Oxidizing polishing slurries for low dielectric constant materials

ABSTRACT

An oxidizing slurry for removal of low dielectric constant materials. The slurry is formed utilizing non-oxidizing particles with a separate oxidizing agent, oxidizing particles alone or reducible abrasive particles with a compatible oxidizing agent. The particles can be formed of a metal oxide, nitride, or carbide material, by itself or mixtures thereof, or can be coated on a core material such as silicon dioxide or can be coformed therewith. A preferred oxidizing slurry is multi-modal in particle size distribution. Although developed for utilization in CMP semiconductor processing the oxidizing slurry of the present invention also can be utilized for other high precision polishing processes.

This application is a divisional of application Ser. No. 09/160,514,filed Sep. 24, 1998 U.S. Pat. No. 6,270,395.

BACKGROUND

1. Field of the Invention

This invention relates to slurries utilized in polishing systems andmore particularly to chemical mechanical polishing oxidizing slurriesfor utilization in semiconductor processing with low-dielectric constantmaterials.

2. Description of Related Art

Integrated circuits are fabricated upon a substantially flat,disc-shaped silicon wafer or substrate, hereinafter referred to assilicon wafers. The surface of a silicon wafer is subdivided into aplurality or array of rectangular areas onto which are formedphotolithographic images, generally identical to one another. Through aseries of well-known processing steps, each of the rectangular areaseventually becomes an individual die on the wafer.

Generally the integrated circuit die, especially in very large scaleintegrated semiconductor circuits, are manufactured by depositing andpatterning a conductive layer or layers upon the semiconductor wafer andthen a nonconductive layer formed from an insulator, covering theconductive layer. The insulator is typically a silicon dioxide (SiO₂)dielectric. The layers are stacked upon one another creating a nonplanartopography, typically caused by the nonconductive or dielectric layersbeing formed over the raised conductive lines or other features in theunderlying conductive layer.

As the integrated circuit devices have become more sophisticated andhence more complex, the number of layers stacked upon one anotherincreases and as the number of layers increase, the planarity problemsgenerally increase. Planarizing the layers during the processing of theintegrated circuits thus can become a major problem and a major expensein producing the circuits. The planarity requirements have resulted in anumber of approaches, and most recently, chemical mechanical polishing(CMP) techniques have been utilized to planarize the semiconductorwafers. Successful utilization of CMP would be highly desirable sincethe CMP techniques are less complex compared to the previously utilizedmethods. The CMP techniques typically utilize a polishing block or pador plurality of the blocks or pads with a chemical slurry. The blocksare rubbed against the layer to be planarized with the addition of achemical slurry which aids in obtaining the planarity of thesemiconductor wafer for further processing.

The necessary parameters for polishing the SiO₂ based intermetaldielectric layers have become well known in the semiconductor industryand the chemical and mechanical nature of polishing and wear of the SiO₂based dielectrics have been reasonably well developed. One problem withthe SiO₂ dielectrics, however, is that the dielectric constant isrelatively high, being approximately 3.9, or higher, depending onfactors including residual moisture content. As a result, thecapacitance between the conductive layers is also relatively high, andthis limits the speed (frequency) at which the circuit can operate.Strategies being developed to reduce capacitance include incorporatingmetals with lower resistivity values, such as copper, and providingelectrical isolation with insulating materials having lower dielectricconstants relative to SiO₂. Thus, it would be highly desirable toincorporate a low dielectric constant material into semiconductorstructures while still being able to utilize the conventional CMPsystems for polishing the surface of the resulting dielectric materialduring the semiconductor wafer processing.

As described herein, “low dielectric constant materials” include“organic polymer materials, porous dielectric materials, whether organicor inorganic, and blended or composite organic and inorganic materials,whether porous or not.”Typically these are polymer dielectric materialswhich include unique chemical, mechanical and electricalcharacteristics, including dielectric constant values of less than three(3.0). These low dielectric constant materials can include relativelyhigh organic content materials, both low and high organic contentmaterials with a high level of porosity, relatively low organic contentmaterials based upon silicon oxygen type materials and inorganicmaterials, or materials exhibiting a combination of these properties.The low dielectric constant films can be deposited utilizing a varietyof techniques including chemical vapor deposition (CVD), and spincoating. The organic polymer materials generally are mechanically softand they readily exhibit plastic deformation and hence they can easilybe scratched. In contrast, however, to their mechanical sensitivity,organic polymers are often chemically inert. The combination of thecharacteristics of the polymer dielectric materials makes an aqueousbased polymer CMP process difficult. Incorporating these low dielectricconstant materials into viable submicron fabrication techniques willnecessitate the development of robust CMP processes which Applicantshave discovered are not currently available utilizing the SiO₂-based CMPprocesses.

Conventional polishing abrasives, such as SiO₂ and Al₂O₃, utilized forCMP and related polishing applications in the optical and diskindustries are typically produced by chemical precipitation methods orby flame hydrolysis. In chemical precipitation, individual oxysaltparticles are typically precipitated from aqueous solutions. Therelatively coarse oxysalt particles are filtered, dried, andsubsequently subjected to a thermal process called calcination whichforms the final, finely divided oxide powder. Low calcinationtemperatures produce high surface area oxide powders that consist ofvery small particles. Increasing the calcination temperature typicallyreduces the total surface area of the powder per unit volume with acorresponding increase in particle size.

In flame hydrolysis, chlorinated or silane precursor materials aresubjected to a high temperature, oxyhydrogen flame. Upon entering theflame the precursor reacts with the hydrogen and oxygen, and istransformed to the final oxide product. The particle size, particle sizedistribution, and surface area of the resulting oxide powder can becontrolled by varying the process temperature, the residence time in thereaction chamber, and the relative concentration of the chemicalprecursors. Oxide powders thus formed consist of very small, primaryparticles that are strongly adhered to other primary particles in a3-dimensional network referred to as an aggregate. These aggregates aremechanically robust and are considered irreducible, i.e., they cannot bebroken down to the dimensional scale of the primary particles undernormal use conditions. The aggregates themselves are often entangledwith other aggregates forming agglomerates.

Conventional polishing slurries are derived by incorporating theagglomerated oxide powder into an aqueous suspension with mechanicalagitation. Limited suspension stability is obtained by incorporatingdispersing agents, or adjusting the suspension pH such that asufficiently high zeta potential is realized to impart stability throughcoulombic interactions. Subsequent particle size reduction processesimprove suspension stability and polishing performance by breaking downlarge particle agglomerates.

The metal oxides used as abrasives can be classified into twocategories: chemically active oxides and chemically inactive oxides.Chemically active oxides are those oxides which contain metals which canbe readily reduced under normal conditions of use. Examples of metalswhich form active types of oxides are cerium (Ce), iron (Fe), tin (Sn),and zirconium (Zr). Chemically inactive oxides are those oxides whichcontain a metal that is not readily reduced under normal use conditions,and therefore, can be considered nonreactive. Examples of these oxidesare aluminum oxide (Al₂O₃), and silicon dioxide (SiO₂).

CMP slurries tailored for SiO₂ dielectrics, typically incorporate SiO₂abrasives in a high pH aqueous slurry. Current thinking holds that thewater hydrolyzes the silicon dioxide material at the silicondioxide/slurry interface, softening it, thus allowing the silicondioxide particles in the slurry to abrade the surface of the dielectric.The high pH environment serves two functions: one, to impart stabilityto the silicon dioxide abrasive slurry and two, to increase thesolubility of the hydrolyzed silicon dioxide groups in the aqueoussolution. Applicants have discovered, however, that as the organiccontent of the film increases, the efficiency of conventional silicondioxide based slurries diminishes rapidly. For example, such an oxideslurry utilized on a typical CMP device, using typical process settings,provides a removal rate of about two thousand five hundred (2500)Angstroms per minute from the surface of the SiO₂ dielectric film.However, these same CMP conditions may only provide a removal rate ofabout two hundred (200) Angstroms per minute on a purely organic polymerfilm. The conversion from mechanical energy to material removal is thusmuch lower and unacceptable for use in semiconductor processing.

One CMP solution for low-dielectric constant materials is disclosed inU.S. patent application Ser. No. 09/096722, entitled “Aqueous MetalOxide Sols As Polishing Slurries For Low Dielectric Constant Materials”,filed Jun. 11, 1998, assigned to the same assignee, including one of thepresent Applicants, which is incorporated herein by reference.

SUMMARY

In accordance with the invention, oxidizing slurries have been developedfor removal of low dielectric constant materials. The slurry is formedin a solution utilizing non-oxidizing abrasive particles with anoxidizing agent, oxidizing abrasive particles alone or selectedoxidizing abrasive particles with compatible oxidizing agents. Theabrasive particles can be formed from a metal oxide, nitride or carbidematerial, by itself or mixtures thereof, or can be coated on a corematerial such as silicon dioxide or can be coformed therewith. Apreferred oxidizing slurry is multi-modal in particle size distribution.Although developed for utilization in CMP semiconductor processing theoxidizing slurry of the present invention also can be utilized for otherprecision polishing processes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an idealized perspective view of a rotary type CMPdevice in which the oxidizing slurry of the present invention can beutilized.

FIG. 2 illustrates an idealized cross section of a semiconductor waferin a CMP process.

FIG. 3 is a graph of results utilizing conventional abrasives on a lowdielectric constant polymer material.

FIG. 4 is a graph of results comparing non-oxidizing particles with andwithout the addition of oxidizing agents forming oxidizing slurryembodiments of the present invention.

FIGS. 5A, 5B and 5C are cross sections of some idealized abrasiveparticle embodiments of the present invention.

FIGS. 6A and 6B are graphs of the removal rate of one abrasive particleembodiment of the present invention with a varying percent of solids andoxidizers.

FIG. 7 illustrates the effect of abrasive solids concentration onremoval rate for different abrasive particles.

FIG. 8 illustrates the effect of the oxidizing slurries on two types ofpads.

FIG. 9 illustrates the CMP removal rate of the low dielectric constantmaterial with entrained abrasive pads.

FIG. 10 illustrates the CMP removal rate with a different abrasive pad.

Utilization of the same reference numerals in different figuresindicates similar or identical elements, structurally and/orfunctionally.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To facilitate description of the oxidizing slurry and the method andapparatus for utilizing the slurry of the present invention, anidealized CMP system or device will be described and is designatedgenerally by the numeral 10 in FIG. 1. The CMP device 10 is a rotarypolishing device. However, the oxidizing slurry of the present inventionalso can be utilized with any type of CMP device which provides relativemotion between the dielectric material to be polished and a polishingsurface, such as orbital or linear CMP devices. The device 10 isdescribed merely to aid in the understanding of a primary use of thepresent invention.

The CMP device 10 is of the type of polishing tool which has amechanical design based upon the semiconductor wafer polishing tools.During CMP of a layer on a semiconductor wafer, the semiconductor wafer(not illustrated) is mounted by well known techniques in a rotary andradially oscillating semiconductor wafer carrier 12. The carrier 12 ismounted to a shaft 14 which imparts the required motion and forces onthe carrier 12 in a conventional manner. The surface of thesemiconductor wafer is pressed against a rotating polishing pad or block16 or array of pads or blocks by a force FT. A chemically abrasiveslurry 18, typically including abrasive particles in a controlled pHsolution, is added to the polishing pad 16 via a supply line or conduit20.

The slurry 18 is added to a wafer track 22, which is an annular ring orzone formed by the rotating polishing pad 16 and the rotating andoscillating carrier 12. The polishing pad 16 is also mounted on a shaft24, which imports the required rotational movement onto the polishingpad 16. The shaft 24 typically is axially fixed, but could also providea portion or all of the force F_(T), if so desired.

The slurry 18 provides both a chemical and a mechanical action on thesurface of the semiconductor wafer to provide a controlled removal ofmaterial from and planarization of the semiconductor wafer surface, aswill be described more fully with respect to FIG. 2. The semiconductorwafer typically is held by a retaining ring or other retaining mechanismin the gimbaled, rotating carrier 14. The polishing pad 16 typically isformed from polyurethane or from polyurethane impregnated fiber,attached, such as by adhesive tape in a conventional manner, to a rigid,temperature controlled base or plate 17. It is believed that during theCMP process, the semiconductor wafer is supported by hydrodynamic forcesand by direct support from the abrasive particles in the slurry 18resting in recesses or deformities in the pad 16 at thepad-semiconductor wafer interface.

The conventional CMP process includes an additional process stepreferred to as pad conditioning. The pad conditioning typically isperformed with a pad conditioner device 26, which typically includes adiamond-impregnated ring or disk tool 27. During the CMP process(“in-situ” conditioning) or just following the polishing process(“ex-situ” conditioning), the conditioner ring or tool 27 is pressedagainst the surface of the polishing pad 16. The pressure applied andthe relative motion (generally radially/oscillating) erodes a smallportion of material from the surface of the polishing pad 16. This paderosion is typically required to maintain the pad surface free ofmaterial build-up associated with the CMP products, such as spentabrasives and dielectric material removed from the surface.

The pad conditioning maintains the micro-texture of the polishing pad16, which tends to smooth out due to viscoelastic flow caused by the CMPprocess. Without pad conditioning, the removal rate and uniformity ofremoval of the semiconductor wafer dielectric material is typically notuniform between semiconductor wafers, hence preventing reliablecommercial production of the semiconductor wafer.

The size of the conditioner ring or tool 27 depends upon the size andthe type of the CMP device 10, but the disk type conditioner 27 usuallyhas a smaller diameter than that of the semiconductor wafer. A ring typeconditioner (not illustrated) generally is larger than the diameter ofthe semiconductor wafer. Therefore, in practice, the ring typeconditioner is positioned at a fixed radial distance from the centralaxis 28 of the polishing pad 16. The ring type conditioner rotates andprovides erosion across the width of the semiconductor wafer track 22.

In the form of a disk type conditioner 27, the disk typically is smallerthan the semiconductor wafer and hence is oscillated across thesemiconductor wafer track 22 to provide the necessary abrasion. Duringthe pad conditioning, the location and rotation rate of the conditioner27 affects the uniformity of erosion in the semiconductor wafer track22, which influences the removal rate stability and the polishinguniformity of the semiconductor wafer surface.

The CMP process continues for a predetermined time to obtain the desiredamount of semiconductor wafer material removal. The predetermined timecalculation is based upon the rate of removal of the semiconductor waferdielectric material and the required amount of dielectric material to beremoved. The removal amount is chosen such that at the end of the CMPprocess, the semiconductor wafer surface has achieved the desiredplanarity and the desired dielectric material thickness.

Typically, the CMP process can utilize a pressure of about 48×10³ Pa(7.0 psi), a velocity of about 0.54 meter per second and a process timeof about three (3) minutes. Referring to FIG. 2, the slurry 18 isillustrated on the polishing pad 16 and includes a plurality of abrasiveparticles 30. The carrier 14 is not illustrated; however, a portion of asemiconductor wafer 32 which is mounted in the carrier 14 isillustrated.

The semiconductor wafer 32 includes a silicon base layer 34, upon whichare formed a plurality of deposited conductive patterned metal features36. The metal features 36 then are covered by a deposited dielectricmaterial layer 38. The layer 38 has a surface 40, which is not planarsince it mirrors the metal features 36 underlying the layer 38. Thesurface 40 then must be planarized before the next photolithography stepcan be performed. The polishing pad 16 and the slurry 18 provide ahigher localized pressure against the regions of higher topography onthe surface 40 to remove these features and planarize the surface 40.

The pad 16 has a hardness and is applied at a force FT (shown by anarrow 42) applied at a velocity V, also shown by an arrow 44. During theCMP process surface regions of high topography on the surface 40, suchas regions or areas 46 are subjected to higher localized contact andhence higher polishing pressures, than regions of low or reducedtopography, such as a region or area 48. The CMP process is designed toselectively remove the regions or areas 46, while minimizing the removalof the regions 48. Assuming the surface of the region 48 is planar andof the right depth, a perfect CMP process would remove all of theregions 46 to be parallel to the surface of the regions 48. Thisselectivity of the CMP process is a measure of the planarizationefficiency of the process.

During the CMP process, the dielectric material 38 is removed by thecombined action of chemical and mechanical processes. The chemicalenergy is supplied by the slurry's liquid media and additionally in somecases by the abrasive particles 30 themselves. CMP slurries which aretailored for removal of dielectric materials typically are aqueous andgenerally have a controlled pH. The mechanical energy is generated bythe relative motion and pressure between the dielectric 38 and thesurface of the pad 16 with the abrasive slurry 18 entrainedtherebetween, as illustrated in FIG. 2.

This relative motion generates mechanical energy, W, in accordance withthe principle of mechanical work;

 W=∫F _(t)μ_(s) ds

In the equation, F_(T) is the force normal to the surface 40, μ_(s) isthe coefficient of sliding friction between the wafer surface 40 and thepad 16 and ds is a differential element of length.

By transferring the differential element of length to a differentialelement of time, the following equation is obtained.

W=∫F _(T)μ_(s) vdt

In this equation, v is the relative velocity between the pad 16 and thedielectric 38. Integration of this equation yields the total workgenerated during the specified time domain. Thus, increasing any of thevariables F_(T), μ_(s), v or the polishing time t increases the materialremoval during the CMP polishing process. As is well known, F. W.Preston recognized the relationship between work and the amount ofmaterial removal and formalized the relationship in an equation, nowknown as the Preston equation, which can be stated in differential formas: $\frac{h}{t} = {K_{p}P\frac{s}{t}}$

In the Preston equation, K_(P) is the Preston coefficient, P is thepolishing pressure acting normal to the surface of the wafer surface 40and ds/dt is the instantaneous relative velocity between the pad 16 andthe surface 40.

As previously mentioned, the CMP process parameters for the conventionalSiO₂-based dielectric layer 38 are well known. To achieve the desireddecrease in the dielectric constant from the 3.9 constant of the SiO₂material, requires a new lower dielectric constant material. Thedevelopment of these dielectric materials has focused on polymer basedmaterials, such as the following:

1. Poly (arylene) Ethers

2. Poly (naphthylene) Ethers

3. Polyimides

4. Poly (benzocyclobutenes) and perfluorocyclobutanes (BCB and PFCB)

5. Polyquinolines

6. Hydrido or Alkylsilsesquioxanes

7. Polytetrofluoroethylene (PTFE)

8. Paralyne-N, Paralyne-F

9. Siloxanes

10. Organic substituted silazanes

11. Polyquinoxalines

12. Various copolymers derived from 1-11.

These dielectric polymer materials include a significant increase in theorganic material content over the SiO₂-based dielectric material. Asalso previously discussed, the conventional CMP conditions and slurriesare much less effective as the organic content increases. Although themajor focus has been on polymer materials, as stated previously, the lowdielectric constant materials can also include porous dielectricmaterials, whether inorganic or organic and mixed organic and inorganicmaterials.

One particular low dielectric constant material which has been utilizedfor testing in accordance with the present invention is apoly(arylene)ether spin-on film, cured in a nitrogen atmosphere,yielding a dielectric constant of about 2.8 when using a voltage signalapplied at one (1) Megahertz. The dielectric material is stable, withoutany substantial outgassing to a processing temperature of approximatelyfour hundred (400) degrees Celsius to make the dielectric materialcompatible with the required back end of the line semiconductor waferprocessing temperature requirements. The dielectric material has aneffective gap fill below 0.12 microns for subtractive aluminum etchprocessing requirements. The conventional SiO₂ slurry utilized in theCMP process with this polymer dielectric material was essentiallyineffective at removing the dielectric material.

Applicants postulated that these types of organic polymer dielectricmaterials, as well as other low dielectic constant materials,effectively will be removed with a mechano-chemical mechanism.Applicants tested this hypothesis by preparing and polishing testblanket and pattern wafers which incorporated a cured spin coated,organic dielectric film. The thermal bake and cure process used toprepare the films is as follows:

Process Step Description 1  3 sec 0 rpm (dispense) 2  5 sec 500 rpm(spread) 3 60 sec 2000 rpm (spin) 4 Bake: 1 min @ 150 Celsius in N₂ 5Cure: 1 hr @ 425 Celsius in N₂

The testing was performed in conventional single-head CMP processingequipment, such as an IPEC 472 made by IPEC Planar Corporation ofPhoenix, Az., a fifteen (15) inch platen table top polisher by EngisCorporation of Wheeling, Ill., and a Teres polisher made by Lam ResearchCorporation of Fremont, Calif.

Referring to FIG. 3, the removal rate of the low dielectric constantpolymer material utilizing some conventional abrasives is illustrated.These abrasives were dispersed in de-ionized (DI) water only and nochemical additives were utilized to enhance their performance. CMP wasperformed for two (2) minutes at a polishing pressure of 23×10³ Pa(approximately 3.3 pounds per square inch (psi)) and a linear velocityof 0.48 meters per second (m/s). The hardest abrasive utilized isdiamond and if the mechanical aspects were solely responsible for thedielectric material removal, then the one hundred (100) or five (500)nanometer (nm) diamond abrasives should have produced the greatest rateof removal. This clearly does not happen. The fumed amorphous SiO₂manufactured by a conventional flame hydrolysis technique produced nodielectric polymer material removal. The chemically active oxideabrasive particles, cerium oxide (CeO₂) and stannic oxide (SnO₂)completely removed all of the dielectric polymer material and repeatedtests proved that the material was stripped within seconds. Thisuncontrollable removal is undesirable, but clearly suggests amechano-chemical removal mechanism.

Applicants then selected and tested a series of specially preparedoxidizing abrasive slurries. Because of the chemical attributes of theseoxidizing slurries, the slurries achieved significantly differentresults, as illustrated in FIG. 4, indicating substantially differentCMP polishing properties. In the present embodiment of this invention,the abrasive particles were formed by well-known flame hydrolysis(fumed) or sol methods. Examples of the sol methods are disclosed inU.S. Pat. Nos. 3,282,857; 4,588,576; 5,004,711 and 5,238,625. Abrasiveslurries derived from these methods can be tailored such that the slurryincludes individual and segregated primary particles, or if so desired,the particle size can be grown to the desired dimension in a repeatable,controlled fashion. The polishing pressure (23×10³ Pa) and the linearvelocity (0.48 m/s) were the same.

Referring to FIG. 4, a fumed TiO₂ or sol SiO₂ particle slurry in DIwater, without an oxidizing agent (no oxidizer) resulted in little or noremoval of the dielectric material. The SiO₂ particles were on the orderof seventy (70) nm in diameter and were dispersed at about two (2) percent solids in DI water to form the slurry. The TiO₂ particles were onthe order of one hundred and seventy-five (175) nm in diameter and weredispersed at about two (2) per cent solids in the DI water.

The TiO₂ particles then were combined with different oxidizing agents toform oxidizing slurry embodiments of the present invention. Whencombined with hydrogen peroxide (H₂O₂), only a small removal rate on theorder of about fifty (50) Angstroms per minute was achieved. Whencombined with Sodium Hypochlorite (NaOCl), a removal rate of just overtwo hundred (200) Angstroms per minute was achieved. A removal rate ofabout nine hundred (900) Angstroms per minute was achieved when the TiO₂particles were combined with ferric nitrate (Fe(NO₃)₃).

The SiO₂ particles then were combined with Fe(NO₃)₃, which resulted in aremoval rate of about nine hundred and fifty (950) Angstroms per minute.Another abrasive particle Al₂O₃ also was combined with Fe(NO₃)₃, whichresulted in a removal rate of about one thousand and fifty (1050)Angstroms per minute. The Al₂O₃ particles had a diameter of aboutseventy (70) mm and were formed as a sol by coating Al₂O₃ over an SiO₂core, as illustrated in FIG. 5B. Clearly, the addition of the oxidizingagent enabled ineffective abrasive particles to be effectively utilizedas a polishing agent in the oxidizing slurries of the present invention.

The particles utilized in the oxidizing slurries of the presentinvention can have several different structures. First, the particlescan be formed as sols and will have a generally spherical shape asillustrated in FIGS. 5A-5C. The particles also can be formed by theconventional flame hydrolysis techniques and then generally would not bespherical, but would be of more irregular shapes. For convenience andease of illustration, the particle structures illustrated in FIGS.5A-5C, are generally spherical; however, the particle structures of thepresent invention are not so limited.

Referring to FIG. 5A, a first particle 50 is illustrated. The particle50, whether formed in solution as a sol and/or calcined, is formed of asubstantially unitary material, such as TiO₂, SiO₂ or Al₂O₃. Referringto FIG. 5B a second particle 52 is illustrated, which includes a corematerial 54 and an outer coating 56. For example, as utilized in FIG. 4,the core 54 can be formed from SiO₂, while the coating 56, can be Al₂O₃.The core material 54 can be chosen for cost, density, ease ofmanufacture and/or other considerations. The density of the particles 52is important, because for a given weight of particles in the oxidizingslurries, the lower density particles will be numerically higher innumber and generally are easier to suspend in the slurry. An increase inthe number of the particles 52 will increase the polishing contact areaand hence generally will increase the removal rate of the dielectricmaterial.

A third composite structure particle 58 also can be formed, asillustrated in FIG. 5C. The particle 58 can be coprecipitated from amixture of materials, such as Al₂O₃ and SiO₂, for example. This resultsin a mixed phase particle having regions 60 of a first materialintermixed with regions 62 of a second material.

The oxidizing slurries of the present invention include an abrasiveparticle and an oxidizing agent. The oxidizing slurries can includeparticles, such as those formed from cerium dioxide (CeO₂), vanadiumpentoxide (V₂O₅), (zirconium oxide (ZrO₂), manganese oxide (Mn₂O₃), tinoxide (SnO₂), antimony pentoxide (Sb₂O₅) or manganese dioxide (MnO₂),which provide both the abrasive and the oxidation functions. Theoxidizing slurries of the present invention also can include abrasiveparticles which provide little or no oxidation, which are combined witha separate oxidizing agent, such as ferric nitrate (Fe(NO₃)₃₎, cupricnitrate (Cu(NO₃)₂), zirconyl nitrate (Zro(NO₃)₂), ferric chloride(FeCl₃), potassium permanganate (KMnO₄), potassium ferricyanide(K₃Fe(CN)₆), nitric acid (HNO₃), potassium iodate (KIO₃), organic andinorganic peroxides, including hydrogen peroxide (H₂O₂), peracetic acid(C₂H₄O₃), and benzoyl peroxide (C₁₄H₁₀O₄) The listed metal salts, forexample, can provide reducible metal ions in the slurry, and these ionsmay be capable of oxidizing the low dielectric constant materials. Theresulting oxidizing slurries also can include pH modifiers andsurfactants, if desired to improve suspension characteristics.

When forming the oxidizing slurries of the present invention, acombination of an abrasive particle and an oxidizing agent or oxidantmust be evaluated for electrochemical compatibility, since somecombinations will form spontaneous electrochemical cells. In anoxidation/reduction (redox) reaction, there is a transfer of electronsfrom the reductant to the oxidant. In the case of the CMP slurry, thepolymer film transfers electrons to the oxidant of the slurry and istherefore converted to a higher valence state, while the oxidant becomesreduced in its valence state. The potential for this reaction to occurcan be described in the form of half-reactions; that is the conversionof one reactant from one valence state to another. The potential forthis half reaction to occur is quantitatively described using areduction potential, which is a voltage relative to a standard hydrogenion/hydrogen reduction, which is defined by convention as having a zerovolt reduction potential.

In order to describe a complete oxidation/reduction reaction, two halfreactions are combined to create a full redox reaction. Taking the sumof the reduction potentials associated with each half reactiondetermines whether the reaction described will be spontaneous, that ishaving a total positive potential, or nonspontaneous, requiring anexternal supply of electrons by having a total negative potential. Forexample, the reaction of reduction of copper (II) ions by metallic zincis described by the following half-reactions:

Zn→Zn²⁺ 2e 0.762 V

Cu²⁺+2e→Cu 0.312 V

Giving an overall reaction of:

Zn+Cu²⁺→Cu+Zn²⁺ 1.074 V

The reduction potentials for a variety of half reactions can be found inany number of chemistry references, such as the CRC Handbook ofChemistry and Physics or Lange's Handbook of Chemistry.

In the case of the CMP slurry described, various transition metals orother compounds having low reduction potentials are used in order toeffect oxidation of the polished surface. With certain metal oxides, theoxide provides both an abrasive action and acts as an oxidant for thepolished surface, or a separate abrasive and oxidant can be used.Because the spontaneity of redox reactions is dependent upon therelative difference between the two half reactions involved, when aslurry is to be prepared using a separate abrasive and oxidant, it isnecessary to determine whether or not the two compounds are compatible;that is, whether they will spontaneously engage in a redox reaction ornot. If they do, the oxidative capacity of the slurry toward thepolished substrate will be substantially reduced or eliminated due toreaction with the abrasive component:

MnO₂+2H⁺+H₂O₂→O₂+Mn²⁺ 0.529 V

2Ce⁴⁺+H₂O₂→O₂+2H⁺+Ce³⁺ 1.025 V

Whereas a nonspontaneous cell is formed as:

ZrO₂2H³⁰+2H₂O₂→Zr+2H₂O+O₂ −2.943 V

Also, only one oxidant should be used, as certain combinations ofoxidants will react with each other, thereby reducing the overalloxidizing capacity of the slurry:

2Fe³⁺+H₂O₂→O₂+2h⁺+2Fe²⁺ 0.847 V

If compatible, the oxidizing reaction aids in the removal of thedielectric material, while if incompatible, the oxidizing reactionoccurs between the particle and the oxidant and does not materially aidin and may prevent the material removal.

Referring to FIGS. 6A and 6B, a graph of the removal rate of oneparticle embodiment of the present invention is illustrated, with avarying percent solids (particles) in FIG. 6A and a varying oxidantconcentration in FIG. 6B. This is a full factorial Design of Experiment(DOE) illustration, which simultaneously examines the role of abrasiveand oxidant concentrations.

In FIG. 6A, a curve 64 illustrates the removal rate results for anincreasing percentage of solids (particles) in the oxidizing slurries ofthe present invention. In this example, the particles are one hundredand seventy-five (175) nm, fumed SiO₂ particles, like the particles 50.The increased number of particles appears to limit the oxidizing effectand in fact does not result in an increased removal rate.

In FIG. 6B, a curve 66 illustrates the removal rate of the SiO₂ particleoxidizing slurries of the present invention as the percentage(concentration) of an oxidizer (Fe(NO₃)₃) is increased in the DIwater/particle solution. The removal rate peaks at about twenty (20)percent volume (about four (4) percent by weight) of the one half (0.5)molar (M) oxidizer in the oxidizing slurries.

Referring to FIG. 7, the removal rate for a first abrasive sol SiO₂particle combined with five (5) percent of an oxidant (Fe(NO₃)₃) atvarying weight percent is shown by a line 68. The same SiO₂ sol particleon the order of seventy (70) mm, combined with ten (10) percent of theoxidant results in a line 70. It is speculated that the decrease in theremoval rate with an increase in the percent solids, is caused bydispersing of the load on each particle along with exclusion of theoxidant/oxidizing slurry fluid, which decreases the removal rate.

Utilization of larger abrasive SiO₂ particles on the order of onehundred seventy-five (175) mm at five (5) percent oxidant concentrationresults in an increased removal rate, line 72. These SiO₂ particles alsoare fumed and hence also may have a more abrasive irregular structurethan the sol SiO₂ particles. Again an increase in the oxidant percent toten (10) results in an increased removal rate, line 74.

Referring to FIG. 8, the results of a ferric nitrate oxidizer solutionof pH 1.5 is illustrated with two different pads, like the pads 16. Acommercially available diamond film type abrasive pad, such as sold byMoyco Technologies, Inc. of Montgomeryville, Pa., under the name type-S(0.1 micron), results in essentially no removal when utilized in D.I.water without an oxidizer. The same pad results in a removal rate ofabout two thousand four hundred (2400) Angstroms per minute with theaddition of the ferric nitrate oxidizer. In contrast a commerciallyavailable non-abrasive IC pad, such as sold by Rodel of Newark, Del.,under the name IC1400, does not include an entrained or surfaceabrasive. Therefore, the removal rate is zero with and without theoxidizer. These results clearly demonstrate the requirement of both anabrasive and an oxidizer or oxidizing agent to achieve a desirableremoval rate for the low dielectric constant material.

FIG. 9 illustrates the removal rate with and without oxidizer forseveral different abrasive entrained pads again, like the pads 16. Afirst abrasive entrained pad, including SiO₂ particles, such as made byUniversal Photonics, Inc. Hicksville, N.Y., under the name LP-99, withDI water but without an oxidizer resulted in an essentially zero removalrate. The addition of six (6) percent by weight of an oxidizer(Fe(NO₃)₃) increased the removal rate to the order of seventeen (17)Angstroms per minute. A second CeO₂ entrained pad, including CeO₂particles, such as made by Universal Photonics, Inc., under the nameTLP-88, in DI water but without a separate oxidizer, resulted in aremoval rate of about eighty-four (84) Angstroms per minute. Addition ofsix (6) percent by weight of the oxidizer (Fe(NO₃)₃) resulted in adecreased removal rate of about sixty-four (64) Angstroms per minute. Inthis case, the oxidizer clearly is competing/interfering with theoxidizer reaction of the CeO₂ particles alone. A third ZrO₂ entrainedpad, including ZrO₂ particles, such as made by Universal Photonics,Inc., under the name GR-35, with DI water, but without a separateoxidizer resulted in a removal rate of about seventy (70) Angstroms perminute. The addition of the Fe(NO₃)₃ oxidizer appears to be compatiblewith the ZrO₂ oxidizer and results in an increased removal rate of abouteighty-one (81) Angstroms per minute. These removal rates are too low tobe commercially viable, but illustrate the oxidizer effect on compatibleand non-compatible abrasive particles. The entrained abrasive pads maybe useful when combined with a non-entrained abrasive slurry of thepresent invention.

Referring to FIG. 10, an experimental pad made by 3M Corporation, havingone half (0.5) micron CeO₂ particle abrasives adhered to the pad surface(like sandpaper) results in a removal rate of over twelve hundred (1200)Angstroms per minute with only DI water. This result clearlydemonstrates the fact that the CeO₂ abrasive forms both the abrasive andthe oxidizing function necessary to obtain removal of the dielectricmaterial.

Although the tests so far indicate that introducing the abrasiveparticles to the surface of the dielectric polymer material incombination with an oxidizer is optimal, other fixed abrasiveembodiments could be utilized such as that illustrated in FIGS. 8-10.The abrasive could be in a gel form and could be bonded directly to thesurface of or incorporated within a suitably thick compliant polishingsubstrate in a separate manufacturing process. This potentially couldeliminate the liquid oxidizing slurry, since the abrasive coatedcompliant substrate could act as the fixed abrasive polishing pad withthe oxidizer in the chemically active particles. The technique couldalso be extended to incorporate the particles into a polymeric matrixwithin the bulk of or at the surface of the polishing pad 16. Acombination of these approaches may be optimal for some applications.

For the abrasive particles of the present invention, the diameter rangefor polishing the low dielectric constant materials is on the order ofthree (3) nanometers (nm) up to one thousand (1000) nm (or one micron),preferably on the order of fifty (50) nm to two hundred and fifty (250)nm. The concentration of the particles in the slurry solution by weightpercent (wt %) is from about one tenth (0.1) to thirty (30) percent;preferably two (2) to fifteen (15) percent.

The particle slurry of the present invention preferably is formed havinga multi-modal particle size distribution, which includes one or moresets of small diameter particles combined with one or more sets oflarger diameter particles. U.S. Pat. No. 5,527,370 discloses that abi-modal particle size distribution provides a higher grinding rate andbetter surface finish for metals and inorganic materials than amono-disperse abrasive system. Applicants believe this also to be thecase for polishing the low dielectric constant materials describedherein.

The abrasive particles of the present invention can be formed fromcarbon or diamond or one or more of the carbides, nitrides, oxides orhydrated oxides of the following metals:

antimony, aluminum, boron, calcium, cerium, chromium, copper,gadolinium, germanium, hafnium, indium, iron, lanthanum, lead,magnesium, manganese, neodymium, nickel, scandium, silicon, terbium,tin, titanium, tungsten, vanadium, yttrium, zinc, or zirconium.

The particles are not necessarily one hundred percent pure and can alsopurposely be formed from a combination of the metal oxides, hydratedoxides, carbides, or nitrides. The core 54 essentially can be anymaterial which can be coated with the abrasive materials of the presentinvention. Small amounts of other materials also can be utilized, ifdesired. The inactive oxide core 54 or mixture 60 can be SiO₂ or can beother oxides or other low density materials as desired.

The present invention thus specifically encompasses abrasive particleslurries with an oxidizing agent, which can be provided by:

(1) Inactive abrasive particle combined with a separate oxidizing agent,such as SiO₂ and Fe(NO₃)₃.

(2) Active reducible abrasive particles with multiple valence states,which provide both the abrasive particle and the oxidizing agent, suchas CeO₂ and SnO₂.

(3) Reducible abrasive particles combined with a compatible oxidizingagent, such as ZrO₂ and HNO₃.

Although the present invention has been described with reference toparticular embodiments, the described embodiments are examples of thepresent invention and should not be taken as limitations. For example,the abrasive particles of the present invention have been described asbeing preferably maintained in a suspension or dispersion; however, theparticles could be in gel form and the particles also could be adheredto or be formed as part of the surface of the pad 16 (not illustrated).A combination of these features also could be utilized. Further althoughspecific abrasive particles have been described, virtually any particlewhich is harder than the low dielectric constant material effectivelycan be utilized with an oxidizing agent. Also, although the pad 16 hasbeen illustrated as a rigid platen mounted polishing pad, it can includeany semi-rigid surface, such as formed on or part of a rotating sphere,rod or cylinder, belt or pad, etc. (not illustrated) As will beappreciated by those skilled in the art, various other adaptations andcombinations of the embodiments described herein are within the scope ofthe present invention as defined by the attached claims.

We claim:
 1. An abrasive composition for polishing polymer-basedmaterials or materials having a dielectric constant value less than 3,the composition comprising: an oxidizing slurry that comprises aplurality of oxidizing abrasive particles, wherein the oxidizingabrasive particles are chemically reactive with the polymer-basedmaterials or the dielectric material to aid in the removal of thepolymer-based materials or the dielectric material; and an additionaloxidizing agent.
 2. The abrasive composition of claim 1, wherein theoxidizing abrasive particles include multiple valence states.
 3. Theabrasive composition of claim 1, wherein the oxidizing agent iscompatible with the oxidizing abrasive particles.
 4. The abrasivecomposition of claim 1, wherein the oxidizing abrasive particles areformed from an active metal oxide combined with an inactive oxide. 5.The abrasive composition of claim 4, the metal oxide is formed as acoating over said inactive oxide.
 6. The abrasive composition of claim4, wherein the inactive oxide is SiO₂.
 7. The abrasive composition ofclaim 1, wherein the additional oxidizing agent comprises ferricnitrate, cupric nitrate, zirconyl nitrate, ferric chloride, potassiumpermanganate, potassium ferricyanide, nitric acid, organic or inorganicperoxides, wherein inorganic peroxides comprises hydrogen peroxide,peracetic acid, potassium iodate or benzoyl peroxide.
 8. The abrasivecomposition of claim 1, wherein the oxidizing abrasive particles have amulti-modal size distribution.
 9. The abrasive composition of claim 8,wherein the oxidizing abrasive particles have a bi-modal sizedistribution, including a plurality of small diameter particles and asecond lesser amount of a plurality of large diameter particles.
 10. Theabrasive composition of claim 1, wherein the slurry is formed in adispersion with a pH of about one half to eleven.
 11. The abrasivecomposition of claim 10, wherein the dispersion has a pH of about one tofive.
 12. A The abrasive composition of claim 1, wherein the oxidizingslurry is formed as a liquid dispersion.
 13. The abrasive composition ofclaim 1, wherein the oxidizing agent includes a plurality of reduciblemetal ions which oxidize the dielectric material or the polymer-basedmaterial.
 14. An abrasive composition for polishing polymer-basedmaterials or materials having a dielectric constant value less than 3,the composition comprising: an oxidizing slurry that comprises aplurality of abrasive particles, wherein the abrasive particles comprisecarbon, diamond antimony, boron, calcium, chromium, copper, gadolinium,germanium, hafnium, indium, iron, lanthanum, lead, magnesium, manganese,neodymium, nickel, scandium, terbium, tin, titanium, tungsten, vanadium,yttrium, zinc or zirconium; and wherein the slurry includes an oxidizingagent that is chemically reactive with the polymer-based material or thedielectric material to aid in the removal of the polymer-based materialor the dielectric material.
 15. The abrasive composition of claim 14,wherein the abrasive particles are substantially inactive and theoxidizing agent is a separate agent from the particles.
 16. The abrasivecomposition of claim 14, wherein the abrasive particles include multiplevalence states and are reducible to provide the oxidizing agent.
 17. Theabrasive composition of claim 14, wherein the abrasive particles includemultiple valence states and wherein the oxidizing agent is a separateoxidizing agent compatible with the abrasive particles.
 18. The abrasivecomposition of claim 14, wherein the abrasive particles are formed froman active metal oxide combined with an inactive oxide.
 19. The abrasivecomposition of claim 18, wherein the metal oxide is formed as a coatingover said inactive oxide.
 20. The abrasive composition of claim 18,wherein the inactive oxide is SiO₂.
 21. The abrasive composition ofclaims 14, wherein the oxidizing agent is a separate agent from theparticles and comprises ferric nitrate, cupric nitrate, zirconylnitrate, ferric chloride, potassium permanganate, potassiumferricyanide, nitric acid, organic or inorganic peroxides, wherein theinorganic peroxides comprise hydrogen peroxide, peracetic acid,potassium iodate or benzoyl peroxide.
 22. The abrasive composition ofclaim 14, wherein the abrasive particles have a multi-modal sizedistribution.
 23. The abrasive composition of claim 22, wherein theabrasive particles have a bi-modal size distribution, including aplurality of small diameter particles and a second lesser amount of aplurality of large diameter particles.
 24. The abrasive composition ofclaim 14, wherein the slurry is formed in a dispersion with a pH ofabout one half to eleven.
 25. The abrasive composition of claim 24, saiddispersion has a pH of about one to five.
 26. The abrasive compositionof claim 14, wherein the oxidizing slurry is formed as a liquiddispersion.
 27. The abrasive composition of claim 14, wherein theoxidizing agent includes a plurality of reducible metal ions whichoxidize the dielectric material or the polymer-based material.